A heat-vulcanizable silicone rubber compound is a base compound comprising a high-viscosity diorganopolysiloxane (gum) as the primary ingredient and, blended therewith, a silica type reinforcing filler and various additives for imparting various desired properties to the compound. From this compound, final formed products of silicone rubber are manufactured usually by rubber processors, and this process comprises blending a vulcanizing agent with the compound and then heat-curing the resulting compound. As such vulcanizing agents, conventional vulcanizing agents such as organic peroxides and addition reaction-type crosslinking agents composed of a combination of an organohydrogen polysiloxane and a platinum compound are used.
Conventional method for the production of such heat-vulcanizable silicone rubber compounds is to uniformly knead a high-viscosity diorganopolysiloxane gum, an inorganic filler and various additives by means of a kneading machine such as a twin-arm kneader (dough mixer), an internal mixer (Banbury mixer) or a two-roll mill. Of such apparatuses, kneaders are most frequently employed. The dough mixer comprises a large tank equipped therein with two large mixing blades which knead the gum and the filler into a homogeneous mixture (compound). In some cases, the above kneading is conducted under heating for the purpose of diminishing the crepe hardening (plasticization reverse) of the compound produced with passage of time. Therefore, the time necessary for a kneader having a capacity of 2 tons to produce such a compound is from a minimum of 6 hours to a maximum of as much as 48 hours. In addition, the thus-obtained compound mass is forced to pass through a filtering extruder to remove foreign particles which have come into the mass, thereby giving a final compound.
Thus, the generally employed process for the production of heat-vulcanizable silicone rubber compounds requires large-sized equipments and much time. Therefore, in order to eliminate these disadvantages, efficient and continuous processes have so far been proposed.
For example, JP-A-61-40327 discloses a process for producing a liquid silicone rubber base by kneading an organopolysiloxane and an inorganic filler, as main ingredients, by means of a twin-screw continuous extruder. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) The organopolysiloxane preferably used in the above process has a specific viscosity range of from 300 to 30,000 cP at 25.degree. C. and shows a good flowability, so that it has excellent compatibility with inorganic fillers. Therefore, due to its low viscosity, the organopolysiloxane can be easily blended with fillers and other additives introduced into a liquid silicone rubber base using only one twin-screw extruder. However, if an organopolysiloxane having a viscosity higher than 1.times.10.sup.5 cP at 25.degree. C. is used and the kneading of the organopolysiloxane with an inorganic filler, particularly with reinforcing silica having a specific surface area of 50 m.sup.2 /g more, is conducted by means of a twin-screw extruder only, much time is often required to obtain a uniform mixture or the composition of the rubber base varies with part and a rubber base having poor quality is only obtained. Thus, a process for producing a silicone rubber base from a high-viscosity organopolysiloxane by means of a twin-screw extruder has not yet been put to practical use.
With respect to heat-vulcanizable silica rubber mixtures, JP-A-50-25650 discloses a process for producing a heat-vulcanizable silicone rubber mixture from a high-viscosity organopolysiloxane as a main ingredient. More particularly, this process is to produce, in a short time, a free-flowing particulate silicone rubber mixture from an organopolysiloxane having a viscosity of from 1.times.10.sup.5 to 2.times.10.sup.8 cP at 25.degree. C. and a filler selected from reinforcing fillers, extending fillers and mixtures thereof, by a mechanical shearing means using high-speed agitating blades. However, the particulate mixture obtained by this process is defective in long-term stability (storage stability). Illustratively stated, if the particulate mixture is allowed to stand at room temperature for a long period of time, particles thereof adhere to each other to impair the flowability of the mixture, or inversely the particulate composition forms a structure (crepe hardening) and becomes rubbery, so that it cannot be plasticized with rolls. Because of these defects, this silicone rubber mixture has a very limited application range as a heat-vulcanizable silicone rubber composition.
The particulate silicone rubber mixture prepared by the above prior art process is advantageous in that if the silicone rubber mixture already contains a vulcanizing agent and is prepared just before, the mixture can be directly vulcanized in an injection molder or an extruder to give a silicone rubber. However, if the particulate silicone rubber mixture contains no vulcanizing agent, the particulate mixture should first be formed into a mass by means of rolls or the like, a vulcanizing agent is added to the mass and a silicone rubber is then formed therefrom. In this case, the step of forming the particulate mixture into a mass is disadvantageous in that the step is not easy and requires much time. Thus, the above process is not always efficient.